Pump and actuator

ABSTRACT

A first surface of a porous medium is in contact with a first space. A second surface of the porous medium is in contact with a second space. The first space is in communication with the second space via the pores of the porous medium. Light in a wavelength range where the material of the porous medium exhibits light absorption properties is applied to the first surface of the porous medium from a light source, so that energy of the applied light can be efficiently absorbed into the porous medium from its first surface, and the first surface of the porous medium can be efficiently heated. As a result, a thermal transpiration flow can be efficiently generated in the porous medium, and a pressure difference can be efficiently produced between the first space and the second space.

PRIORITY INFORMATION

This application claims priorities to Japanese Patent Application Nos. 2012-185076, filed on Aug. 24, 2012, 2013-020589, filed on Feb. 5, 2013, and 2013-151827, filed on Jul. 22, 2013, which are incorporated herein by reference in their entireties.

BACKGROUND

1. Technical Field

The present invention relates to a pump and an actuator that use a thermal transpiration flow.

2. Related Art

A thermal transpiration flow is the flow specific to a rarefied gas. When there is a wall having a temperature gradient in the rarefied gas, a flow in a direction along the wall is induced from a low-temperature part to a high-temperature part. This is referred to as the thermal transpiration flow. The rarefied gas is a gas which has such few impacts between gas molecules that an equilibrium state is not maintained in a given region. A specific example of the rarefied gas has a low pressure of about 1 Pa in a region of about 1 cm³, or has a pressure substantially equal to atmospheric pressure in a small region having a space of about 10×10×10 nm. As in the latter case, the rarefied gas is formed in a region having a small scale even under atmospheric pressure, and the thermal transpiration flow can be generated even in a condition substantially at atmospheric pressure.

A porous medium in which a large number of pores having a small diameter equal to or less than a length that is five times the mean free path (about 60 nm under atmospheric pressure) of an ambient gas are formed is used to generate a thermal transpiration flow in the condition substantially at atmospheric pressure. According to N. K. Gupta et al., “Thermal transpiration in mixed cellulose ester membranes: Enabling miniature, motionless gas pumps” (hereinafter, Nonpatent Document 1), air on one side of a porous membrane is heated by a heater, and one surface of the porous membrane is thereby indirectly heated, in order to generate a thermal transpiration flow in the porous membrane. In this way, a temperature difference is produced between one side of the porous membrane and its rear side, and a temperature gradient is generated in the porous membrane.

To generate a thermal transpiration flow in the porous medium, it is necessary to generate a temperature gradient in the porous medium. According to Nonpatent Document 1, air on one side of the porous membrane is heated by the heater to generate a temperature gradient in the porous membrane. However, still air has low thermal conductivity (0.02 [W/(m·K)]), and the heat of the heater is not easily transmitted to the one side of the target porous membrane, so that the heating efficiency is low. Therefore, according to Nonpatent Document 1, it is difficult to efficiently generate a thermal transpiration flow in the porous membrane.

SUMMARY

An advantage of the present invention is to efficiently generate a thermal transpiration flow in a pump and an actuator that use a thermal transpiration flow.

The pump that uses the thermal transpiration flow can transfer a gas, but cannot transfer a liquid, in principle. Another advantage of the present invention is to enable the transfer of a liquid in the pump that uses the thermal transpiration flow.

A pump according to the present invention includes a porous medium which has light absorption properties in a predetermined wavelength range and which has pores, and a heating unit which applies electromagnetic waves having a peak of radiant intensity in the predetermined wavelength range to a first surface of the porous medium to heat the first surface of the porous medium, wherein the first surface of the porous medium is in contact with a first space, a second surface of the porous medium different from the first surface is in contact with a second space, the first space is in communication with the second space via the pores, and a gas in the second space can be transferred to the first space via the communication pores.

In one aspect of the present invention, it is preferable that the pores of the porous medium have a diameter equal to or less than a length that is five times the mean free path of the gas filling the first space or the second space.

In one aspect of the present invention, it is preferable that the electromagnetic waves are applied to the first surface of the porous medium via a transmission window having light transmission properties in the predetermined wavelength range.

In one aspect of the present invention, it is preferable that the pump further comprises a heat radiation device for heat radiation in the second surface of the porous medium.

In one aspect of the present invention, it is preferable that the porous medium has a peak of light absorption properties for infrared light and that the heating unit applies electromagnetic waves having a peak of radiant intensity in an infrared light region to the first surface of the porous medium.

In one aspect of the present invention, it is preferable that the porous medium has a peak of light absorption properties for visible light and that the heating unit applies electromagnetic waves having a peak of radiant intensity in a visible light region to the first surface of the porous medium.

In one aspect of the present invention, it is preferable that the porous medium is made of silicon dioxide and that the predetermined wavelength range is 2.3 μm or more.

In one aspect of the present invention, it is preferable that the porous medium is made of silicon dioxide and that the predetermined wavelength range is 2.3 to 4.0 μm.

In one aspect of the present invention, it is preferable that the porous medium is made of silicon dioxide and that the predetermined wavelength range is 2.5 to 3.8 μm or is 4.8 μm or more.

In one aspect of the present invention, it is preferable that gas molecules in the second space are transferred to the first space by a thermal transpiration flow generated in the pores.

An actuator according to the present invention includes a drive unit based on a pressure difference produced between the first space and the second space by the pump according to the present invention.

In one aspect of the present invention, it is preferable that the first space is in communication with a third space having a liquid discharge opening and that the pump is configured to pressurize the third space to discharge a liquid in the third space from the liquid discharge opening.

In one aspect of the present invention, it is preferable that the second space is in communication with a fourth space having a liquid suction opening and that the pump is configured to depressurize the fourth space to suck a liquid into the fourth space from the liquid suction opening.

In one aspect of the present invention, it is preferable that the pump further includes a first valve which permits or cuts off the communication between a fifth space having a liquid transfer opening and the first space, and a second valve which permits or cuts off the communication between the fifth space and the second space, and the pump has a discharge unit to discharge a liquid in the fifth space from the liquid transfer opening by using the first valve to permit the communication between the fifth space and the first space and using the second valve to cut off the communication between the fifth space and the second space, and a suction unit to suck a liquid into the fifth space from the liquid transfer opening by using the first valve to cut off the communication between the fifth space and the first space and using the second valve to permit the communication between the fifth space and the second space.

A pump according to the present invention includes a porous medium having pores, and a device which brings the temperature of a first surface of the porous medium to a temperature higher than the temperature of a second surface of the porous medium different from the first surface, wherein the first surface of the porous medium is in contact with a first space, the second surface of the porous medium different from the first surface is in contact with a second space, the first space is in communication with the second space via the pores, a gas in the second space can be transferred to the first space via the communication pores, the first space is in communication with a third space having a liquid discharge opening, and a liquid in the third space can be discharged from the liquid discharge opening by the pressurization of the third space.

A pump according to the present invention includes a porous medium having pores, and a device which brings the temperature of a first surface of the porous medium to a temperature higher than the temperature of a second surface of the porous medium different from the first surface, wherein the first surface of the porous medium is in contact with a first space, the second surface of the porous medium different from the first surface is in contact with a second space, the first space is in communication with the second space via the pores, a gas in the second space can be transferred to the first space via the communication pores, the second space is in communication with a fourth space having a liquid suction opening, and a liquid is capable of being sucked into the fourth space from the liquid suction opening by the depressurization of the fourth space.

An electricity generating equipment is configured to generate electricity by using the pump according to the present invention.

In one aspect of the present invention, it is preferable that the first surface is one main surface of the porous medium and that the second surface is a surface of the porous medium opposite to the one main surface.

According to the present invention, electromagnetic waves including the wavelength range where the porous medium has the light absorption properties are applied to one main surface of the porous medium, so that energy of the applied electromagnetic waves can be efficiently absorbed into the porous medium from the one main surface thereof, and the one main surface of the porous medium can be efficiently heated. As a result, a thermal transpiration flow can be efficiently generated in the porous medium, and a pressure difference can be efficiently produced.

Furthermore, according to the present invention, the thermal transpiration flow generated in the porous medium can be used to pressurize the third space which is in communication with the first space to discharge the liquid into the third space, or to depressurize the fourth space which is in communication with the second space to suck the liquid into the fourth space. Consequently, the thermal transpiration flow generated in the porous medium can be used to transfer the liquid.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiment(s) of the present invention will be described in detail based on the following figures, wherein:

FIG. 1 is a diagram showing a general configuration of a pump according to an embodiment of the present invention;

FIG. 2A is a graph showing a light transmission spectrum of silicon dioxide;

FIG. 2B is a graph showing a black body radiation spectrum of a substance at 900° C.;

FIG. 3 is a diagram showing another general configuration of the pump according to the embodiment of the present invention;

FIG. 4A is a graph showing a light transmission spectrum of calcium fluoride;

FIG. 4B is a graph showing a light transmission spectrum of sodium chloride;

FIG. 4C is a graph showing experimental data regarding the measurement of a light transmission spectrum of silica aerogel;

FIG. 5 is a diagram showing another general configuration of the pump according to the embodiment of the present invention;

FIG. 6 is a diagram showing a general configuration of an actuator according to the embodiment of the present invention;

FIG. 7 is a diagram showing another general configuration of the pump according to the embodiment of the present invention;

FIG. 8 is a diagram showing another general configuration of the pump according to the embodiment of the present invention;

FIG. 9A is a diagram showing another general configuration of the pump according to the embodiment of the present invention;

FIG. 9B is a diagram showing another general configuration of the pump according to the embodiment of the present invention;

FIG. 10 is a graph showing an example of the relation between energy applied to the surface of a porous medium and the transfer flow volume of a liquid in the pump according to the embodiment of the present invention;

FIG. 11 is a diagram showing another general configuration of the pump according to the embodiment of the present invention;

FIG. 12 is a diagram showing a general configuration of a cooling device according to the embodiment of the present invention;

FIG. 13 is a diagram showing a general configuration of the cooling device according to the embodiment of the present invention;

FIG. 14 is a diagram showing a general configuration of an electricity generating equipment according to the embodiment of the present invention; and

FIG. 15 is a diagram showing a general configuration of the electricity generating equipment according to the embodiment of the present invention.

DETAILED DESCRIPTION

Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

FIG. 1 is a diagram showing a general configuration of a pump according to the embodiment of the present invention.

The pump according to the present embodiment is a thermally driven molecular flow pump which can pressurize or depressurize a gas by using a thermal transpiration flow.

A porous membrane 10 disposed in a casing 20 is made of a material having a low thermal conductivity (0.2 [W/(m·K)] or less) such as a silica aerogel membrane in which a large number of pores 12 are formed in a silicon dioxide (silica, SiO₂) material. A first space 16 is formed between the casing 20 and a front surface (first surface, one main surface) 10 a of the porous membrane 10. A second space 18 is formed between a heat sink (heat radiation device) 14 disposed in the casing 20 and a rear surface (opposite second surface different from the first surface (one main surface)) 10 b of the porous membrane 10. That is, the front surface 10 a of the porous membrane 10 is in contact with the first space 16, and the rear surface 10 b of the porous membrane 10 is in contact with the second space 18. Around the second space 18 between the heat sink 14 and the rear surface 10 b of the porous membrane 10, an airtight seal 22 is provided in close contact with the heat sink 14 and the rear surface 10 b of the porous membrane 10. The second space 18 is in communication with an inflow opening 17 via an inflow path 19 formed in the heat sink 14. The first space 16 is in communication with an outflow opening 15. Further, the second space 18 and the first space 16 are in communication with each other via the large number of pores 12 in the porous membrane 10. Each of the pores 12 in the porous membrane 10 is formed to have a diameter equal to or less than a length that is five times the mean free path (about GO nm under the atmospheric pressure) of an ambient gas filling the first space 16 or the second space 18, for example, formed to have a diameter of about 10 nm.

A light source 24 is disposed between the casing 20 and the front surface 10 a of the porous membrane 10 (in the first space 16) to apply electromagnetic waves (light) 44 to the front surface 10 a of the porous membrane 10. The light source 24 is supported on the casing 20 via a support member 25. The light source 24 here is an infrared light source which applies infrared light as the light 44 to the front surface 10 a of the porous membrane 10. For example, an infrared heater can be used as the infrared light source.

A light transmission spectrum of silicon dioxide which is the material of the porous membrane 10 is shown in FIG. 2A. As shown in FIG. 2A, silicon dioxide exhibits a high light transmittance to light having a wavelength shorter than 2.3 μm, but exhibits a lower light transmittance to light having a wavelength of 2.3 μm or more. Thus, silicon dioxide exhibits light transmission properties in a wavelength range shorter than 2.3 μm, for example, light transmission properties in the wavelength range of a visible light region. In contrast, silicon dioxide exhibits light absorption properties in a wavelength range of 2.3 μm or more, for example, light absorption properties in the wavelength range of an infrared light region, and has a peak of the light absorption properties for infrared light. Therefore, the electromagnetic waves (light) 44 including a wavelength range of 2.3 μm or more where the silicon dioxide material of the porous membrane 10 exhibits light absorption properties are applied to the front surface 10 a of the porous membrane 10 from the light source 24. As a result, energy of the applied light 44 is absorbed into the porous membrane 10 from its front surface 10 a. For example, the infrared light 44 having a light absorption wavelength range of 2.3 μm to 4.0 μm is applied to the front surface 10 a of the porous membrane 10, and energy of the applied light 44 is partly absorbed into the porous membrane 10 from its front surface 10 a. Thus, the light source 24 can be used as a heating unit to heat the front surface 10 a of the porous membrane 10.

A black body radiation spectrum of a substance at 900° C. is shown in FIG. 2B. As shown in FIG. 2B, the black body radiation spectrum at 900° C. has the maximum irradiance (intensity) at a wavelength of about 2.5 μm, and exhibits a high irradiance in the wavelength range of an infrared light region of 2.3 μm to 4.0 μm. Therefore, the black body radiation spectrum at 900° C. has a peak of irradiance in a wavelength range of 2.3 μm or more where silicon dioxide exhibits light absorption properties, and a light emission wavelength range of 2.3 μm to 4.0 μm exhibiting a high irradiance corresponds to the light absorption wavelength range of silicon dioxide. Moreover, the black body radiation spectrum changes with temperature, and the irradiance increases with the temperature rise, and the wavelength which maximizes the irradiance is reduced. Thus, the filament temperature of the light source 24 is increased and adjusted to a temperature (e.g. 900° C.) satisfying a condition in which the black body radiation spectrum has a peak in a wavelength range of 2.3 μm or more where silicon dioxide exhibits light absorption properties. As a result, the light source 24 can emit, to the front surface 10 a of the porous membrane 10, the electromagnetic waves (light) 44 having a peak of intensity in a wavelength range (about 2.5 μm when the filament temperature is 900° C.) where silicon dioxide exhibits light absorption properties. This allows the light emission wavelength range at which the irradiance of the light source 24 is high to be set to the light absorption wavelength range of silicon dioxide. The filament temperature of the light source 24 can be adjusted by the adjustment of power supplied to the light source 24.

When the thermally driven molecular flow pump according to the present embodiment is actuated, the filament temperature of the light source 24 is increased and adjusted to a temperature (e.g. 900° C.) satisfying the condition in which the black body radiation spectrum has a peak in the wavelength range of the infrared light region where the silicon dioxide material of the porous membrane 10 exhibits light absorption properties. As a result, the electromagnetic waves (infrared light) 44 having a peak of intensity in the wavelength range of the infrared light region where the silicon dioxide material exhibits light absorption properties are emitted from the light source 24, and energy of the emitted infrared light 44 is absorbed into the porous membrane 10 from its front surface 10 a. Thus, if the front surface 10 a of the porous membrane 10 is heated, the temperature of the front surface 10 a of the porous membrane 10 becomes higher than the temperature of the rear surface 10 b. Accordingly, a temperature difference is produced, and a temperature gradient in a thickness direction is generated in the porous membrane 10. In response to the generation of the temperature gradient, a thermal transpiration flow 40 from the second space 18 (low-temperature side) to the first space 16 (high-temperature side) is generated in the porous membrane 10. At the same time, the heat in the rear surface 10 b of the porous membrane 10 is radiated by the heat sink 14. As a result, the temperature difference between the front surface 10 a and the rear surface 10 b of the porous membrane 10 can be increased to increase the temperature gradient in the porous membrane 10, and the thermal transpiration flow 40 which passes through the pores 12 in the porous membrane 10 can be increased. In response to the generation of the thermal transpiration flow 40, gas molecules in the second space 18 facing the rear surface 10 b of the porous membrane 10 are transferred to the first space 16 facing the front surface 10 a via the pores 12 in the porous membrane 10, and the pressure of the gas in the first space 16 becomes higher than the pressure of the gas in the second space 18 so that a pressure difference is produced. Therefore, this pressure difference can be used to function as a gas pump for pressurizing or depressurizing a gas. When the second space 18 is at atmospheric pressure (open to the atmosphere), the gas in the first space 16 is pressurized by the thermal transpiration flow 40 compared with the atmospheric pressure, which allows a function as a pressurization pump. On the other hand, when the first space 16 is at atmospheric pressure (open to the atmosphere), the gas in the second space 18 is depressurized by the thermal transpiration flow 40 compared with the atmospheric pressure, which allows a function as a depressurization pump.

To generate the thermal transpiration flow 40 from the second space 18 to the first space 16 in the porous membrane 10, it is necessary to heat the front surface 10 a of the porous membrane 10 to generate a temperature gradient in a thickness direction in the porous membrane 10. According to the present embodiment, the light emission wavelength range at which the irradiance of the light source 24 is high is set to the light absorption wavelength range of the silicon dioxide material of the porous membrane 10 (the wavelength range of the infrared light region). As a result, energy of the light 44 (infrared light) emitted from the light source 24 can be efficiently absorbed into the porous membrane 10 from its front surface 10 a, and the front surface 10 a of the porous membrane 10 can be efficiently heated in a non-contact state. In this case, the energy of the light 44 is absorbed into the porous membrane 10, so that no surface treatment of the front surface 10 a of the porous membrane 10 is needed to, for example, form a membrane for absorbing the light 44, and the pores 12 in the porous membrane 10 are not blocked. Therefore, according to the present embodiment, the thermal transpiration flow 40 from the second space 18 to the first space 16 can be efficiently generated in the porous membrane 10, and a pressure difference can be efficiently produced between the first space 16 and the second space 18. As a result, it is possible to obtain a thermally driven molecular flow pump having high performance with respect to flow volume and pump head.

Infrared light generally includes light having all wavelengths of 0.7 μm or more. Therefore, when infrared light having a peak of irradiance, for example, at 1 μm is emitted from the light source 24, light of about 1 μm having a high irradiance is out of the light absorption wavelength range of the silicon dioxide material of the porous membrane 10 and therefore passes through the porous membrane 10 and does not contribute to the heating of the front surface 10 a of the porous membrane 10. Moreover, if the light which has passed through the porous membrane 10 is absorbed in a substance other than the porous membrane 10, such as the heat sink 14, the temperature gradient in the porous membrane 10 is reduced, and the performance of the thermally driven molecular flow pump deteriorates. In order to efficiently generate the thermal transpiration flow 40 in the porous membrane 10, it is necessary to inhibit the passage of the light 44 emitted from the light source 24 through the porous membrane 10 so that the light 44 contributes to the heating of the front surface 10 a of the porous membrane 10 by examining the light absorption wavelength range of the material of the porous membrane 10 and adjusting the wavelength range of the light 44 emitted from the light source 24 (the temperature of the light source 24) so that the peak of irradiance is present in the light absorption wavelength range.

An experiment was conducted to quantitatively evaluate the power of the thermally driven molecular flow pump according to the present embodiment. An experimental apparatus was configured in which the first space 16 was at atmospheric pressure (open to the atmosphere) and the second space 18 was a closed space in the configuration shown in FIG. 1. The filament of the light source 24 (infrared heater) was heated to 900° C., and the infrared light 44 was thereby applied to the front surface 10 a of the porous membrane 10 (silica aerogel membrane having a thickness of about 1 mm and a porosity of about 90%). The pressure in the second space 18 was then measured. According to the experimental results, the pressure in the second space 18 could be decreased by 25.5 kPa (about 25% of atmospheric pressure) compared with atmospheric pressure. As the maximum value shown in Nonpatent Document 1 is 1.05 kPa, 25.5 kPa according to the present embodiment corresponds to 24 times or more. A liquid can be lifted 1 mm per 10 Pa, so that 25.5 kPa corresponds to a pump head of 2.55 m in terms of pump performance.

According to the present embodiment, it is also possible to apply the light 44 to the front surface 10 a of the porous membrane 10 by the light source 24 other than the infrared heater to heat the front surface 10 a. For example, in the configuration shown in FIG. 5, the light source 24 is made by a light source that uses vacuum-encapsulated carbon or a material having a radiation factor of 0.7 or more which is close to that of a black body. A condensing lens 26 (e.g. Fresnel lens) for condensing sunlight 46 (light having a wavelength of about 0.3 μm to 1.2 μm) is disposed at a position in the casing 20 facing the front surface 10 a of the porous membrane 10. The focus of the condensing lens 26 is located in a space on the front surface 10 a of the porous membrane 10. The light source 24 (e.g. vacuum-encapsulated carbon) is located between the condensing lens 26 and the front surface 10 a of the porous membrane 10, and is also located at the focal position of the condensing lens 26 so that the sunlight 46 condensed by the condensing lens 26 is condensed to the light source 24.

As carbon absorbs substantially 100% of visible light and infrared light, the light source 24 (vacuum-encapsulated carbon) is heated by the absorption of energy of the sunlight 46 condensed by the condensing lens 26. In this case, carbon does not burn out even at high temperature if there is no ambient oxygen as a result of vacuum encapsulation. If the temperature of the light source 24 (vacuum-encapsulated carbon) is increased and adjusted to a temperature (e.g. 900° C.) at which the black body radiation spectrum has a high irradiance in the wavelength range of the infrared light region, the light source 24 can function as an infrared light source which emits, to the front surface 10 a of the porous membrane 10, the infrared light 44 in a wavelength range where the silicon dioxide material of the porous membrane 10 exhibits light absorption properties. Thus, the condensing lens 26 and the light source 24 are used as a heating unit so that the front surface 10 a of the porous membrane 10 can be efficiently heated in a non-contact state, and the thermal transpiration flow 40 from the second space 18 to the first space 16 can be efficiently generated in the porous membrane 10. Moreover, in the configuration example shown in FIG. 5, the energy of the sunlight 46 is used as the energy for heating the front surface 10 a of the porous membrane 10, so that energy other than the sunlight 46, such as electrical energy, is not needed to drive the thermally driven molecular flow pump, and energy efficiency can therefore be higher. However, in the configuration example shown in FIG. 5, visible light energy other than the sunlight 46 can also be used as the energy for heating the light source 24 (vacuum-encapsulated carbon). In this case, the light source 24 (vacuum-encapsulated carbon) is heated by the visible light energy and thereby functions as an infrared light source which emits the light 44 to the front surface 10 a of the porous membrane 10.

According to the present embodiment, it is also possible to provide a light transmission window 27 between the light source 24 and the front surface 10 a of the porous membrane 10, for example, as shown in FIG. 3, to seal in the porous membrane 10 from the light source 24. The light transmission window 27 exhibits light transmission properties for the wavelength range of the electromagnetic waves (light) 44 emitted from the light source 24. That is, the light transmission window 27 exhibits light transmission properties (does not exhibit light absorption properties) for the wavelength range where the material of the porous membrane 10 has light absorption properties, and has a light absorption wavelength range different from the emission wavelength range of the light 44 from the light source 24 and the light absorption wavelength range of the material of the porous membrane 10. Therefore, the light 44 from the light source 24 is applied to the front surface 10 a of the porous membrane 10 via the light transmission window 27, and the front surface 10 a of the porous membrane 10 is heated. In the example in which the material of the porous membrane 10 is silicon dioxide, calcium fluoride (CaF₂) having a light transmission spectrum shown in FIG. 4A and sodium chloride (NaCl) having a light transmission spectrum shown in FIG. 4B can be shown as the examples of the material of the light transmission window 27. For example, the infrared light 44 in a wavelength range of 2.3 μm to 4.0 μm is applied to the front surface 10 a of the porous membrane 10 through the calcium fluoride or sodium chloride light transmission window 27.

When, for example, the light transmission window 27 and the porous membrane 10 are made of the same material (e.g. silicon dioxide), the light transmission window 27 and the porous membrane 10 correspond to each other in light transmission wavelength range and light absorption wavelength range. In this case, even if the light (e.g. infrared light of 2.3 μm to 4.0 μm) having the light absorption wavelength range of the porous membrane 10 is emitted from the light source 24, the light hardly passes through the light transmission window 27 and is absorbed therein, and does not contribute to the heating of the front surface 10 a of the porous membrane 10. In order to efficiently generate the thermal transpiration flow 40 in the porous membrane 10, it is necessary to inhibit the passage of the light 44 emitted from the light source 24 through the light transmission window 27 and through the porous membrane 10 so that the light 44 contributes to the heating of the front surface 10 a of the porous membrane 10. To this end, different materials are used for the light transmission window 27 and the porous membrane 10 so that the light transmission wavelength range of the light transmission window 27 and the light absorption wavelength range of the porous membrane 10 have a common wavelength range, and the wavelength range of the light 44 emitted from the light source 24 (the temperature of the light source 24) is adjusted so that the peak of irradiance is present in the light absorption wavelength range of the porous membrane 10 and the light transmission wavelength range of the light transmission window 27 (common wavelength range).

In the embodiment described above, even if the temperature of the light source 24 is not 900° C., it is possible to satisfy the condition that has the peak of the black body radiation spectrum in the wavelength range of the infrared light region where the silicon dioxide material of the porous membrane 10 exhibits light absorption properties, and the light source 24 can emit, to the front surface 10 a of the porous membrane 10, the infrared light 44 having a peak of intensity in the wavelength range of the infrared light region where the silicon dioxide material exhibits light absorption properties. For example, when the temperature of the light source 24 is 1200° C. or less, the black body radiation spectrum has a high irradiance in the wavelength range of the infrared light region, and the infrared light 44 can be emitted to the front surface 10 a of the porous membrane 10. The black body radiation spectrum increases in irradiance with the temperature rise. Therefore, in order to increase the energy absorbed into the porous membrane 10 from the light source 24, it is preferable to raise the temperature of the light source 24 within a range satisfying the condition in which the black body radiation spectrum has a high irradiance in a wavelength range where the silicon dioxide material of the porous membrane 10 exhibits light absorption properties. The light emission wavelength range at which the irradiance (black body radiation spectrum) of the light source 24 is high does not always need to be completely set to the light absorption wavelength range of the material of the porous membrane 10. For example, it is also possible to partly set the light emission wavelength range at which the irradiance of the light source 24 is high to the light absorption wavelength range of the material of the porous membrane 10, or bring the light emission wavelength range at which the irradiance of the light source 24 is high close to the light absorption wavelength range of the material of the porous membrane 10.

Experimental data regarding the measurement of a light transmission spectrum of a product named “SP30” and manufactured by JFCC is shown in FIG. 4C as another example of silica aerogel including silicon dioxide. As shown in FIG. 4C, it is found out that the light absorption properties of silica aerogel are particularly preferable in a wavelength range of 2.5 μm to 3.8 μm and in a wavelength range of 4.8 μl or more. The reasons for this are as follows. The range of 2.5 μm to 3.8 μm is the absorption range of an OH group or an alkyl group, and it is considered that an OH group or an alkyl group included in silicon dioxide, for example, makes a contribution. The range of 4.8 μm or more is the absorption range of silicon dioxide, and it is considered that silicon dioxide makes a contribution. Moreover, it is considered that a general silica aerogel membrane made of silicon dioxide other than the aforementioned product has similar light absorption properties. Thus, in the present embodiment, the electromagnetic waves (infrared light) 44 having a peak of intensity in the wavelength range of the infrared light region of 2.5 μl to 3.8 μm where silica aerogel exhibits light absorption properties are emitted from the light source 24 so that energy of the emitted infrared light 44 can be efficiently absorbed into the porous membrane 10 (silica aerogel membrane) from its front surface 10 a. Therefore, the front surface 10 a of the porous membrane 10 can be efficiently heated with less energy, and the thermal transpiration flow 40 can be efficiently generated. As a result, an efficient thermal transpiration flow pump can be obtained. Alternatively, in the present embodiment, the electromagnetic waves (infrared light) 44 having a peak of intensity in the wavelength range of the infrared light region of 4.8 μm or more where silica aerogel exhibits light absorption properties are emitted from the light source 24. This also allows the front surface 10 a of the porous membrane (silica aerogel membrane) 10 to be efficiently heated with less energy so that the thermal transpiration flow 40 can be efficiently generated.

In the present embodiment, a visible light source (e.g. halogen lamp) having a wavelength of 0.4 μm to 0.8 μm can be used as the light source 24. In this case, a material which exhibits light absorption properties in the wavelength range of the visible light region and which has a peak of the light absorption properties for visible light is used as the material of the porous membrane 10. Although silicon carbide (SiC), for example, can be used as the material of the porous membrane 10 in this case, other black or deep blue materials can also be used. When the visible light source is used as the light source 24, the temperature of the light source 24 (visible light source) is increased and adjusted to a temperature satisfying the condition that has the peak of the black body radiation spectrum in the wavelength range of the visible light region where the material of the porous membrane 10 exhibits light absorption properties. As a result, the light source 24 emits, to the front surface 10 a of the porous membrane 10, the electromagnetic waves (visible light) 44 having a peak of intensity in the wavelength range of the visible light region where the material of the porous membrane 10 exhibits light absorption properties. When the temperature of the light source 24 is higher than 1200° C., the black body radiation spectrum has a high irradiance in the wavelength range of the visible light region, and the visible light 44 can be emitted to the front surface 10 a of the porous membrane 10. The visible light 44 in a wavelength range where the material of the porous membrane 10 exhibits light absorption properties is applied to the front surface 10 a of the porous membrane 10 from the light source 24 so that energy of the applied visible light 44 is absorbed into the porous membrane 10 from its front surface 10 a. Consequently, the front surface 10 a of the porous membrane 10 can be efficiently heated in a non-contact state, and the thermal transpiration flow 40 from the second space 18 to the first space 16 can be efficiently generated in the porous membrane 10.

When a material such as silicon carbide that exhibits light absorption properties in the wavelength range of the visible light region is used as the material of the porous membrane 10, it is also possible to heat the front surface 10 a of the porous membrane 10 by applying the sunlight 46 to the front surface 10 a of the porous membrane 10, and thus generate a thermal transpiration flow from the second space 18 to the first space 16 in the porous membrane 10. The following configuration can be considered in this case: The light source 24 (vacuum-encapsulated carbon) is omitted in the configuration shown in FIG. 5, and the sunlight 46 condensed by the condensing lens 26 is applied to the front surface 10 a of the porous membrane 10.

In the embodiment described above, the heat sink 14 for heat radiation in the rear surface 10 b of the porous membrane 10 can be omitted. In this case as well, the thermal transpiration flow 40 from the second space 18 to the first space 16 can be generated in the porous membrane 10.

In the embodiment described above, the pressure difference produced between the first space 16 and the second space 18 by the thermal transpiration flow 40 in the porous membrane 10 can be used to drive a drive unit of an actuator. That is, the configuration according to the embodiment described above can be used to configure the actuator. For example, in an actuator having a configuration shown in FIG. 6, a drive unit 34 moves to a low-pressure side when a pressure difference is produced between the first space 16 and the second space 18 by the thermal transpiration flow 40 in the porous membrane 10.

In the embodiment described above, the pressure difference produced between the first space 16 and the second space 18 by the thermal transpiration flow 40 in the porous membrane 10 can be used to move a liquid. That is, the configuration according to the embodiment described above can be used to configure a liquid pump. For example, in a liquid pump having a configuration shown in FIG. 7, the second space 18 is opened to the atmosphere, and the first space 16 is in communication with an inner space 30 a of a liquid tank 30 via a flow path 51. Moreover, a liquid discharge opening 30 b is provided in the liquid tank 30, and the inner space 30 a of the liquid tank 30 is in communication with the outside of the liquid tank 30 via the liquid discharge opening 30 b. A liquid 32 is contained in the inner space 30 a of the liquid tank 30. If the gas in the first space 16 is pressurized by the thermal transpiration flow 40 in the porous membrane 10 compared with the atmospheric pressure, the inner space 30 a of the liquid tank 30 which is in communication with the first space 16 via the flow path 51 is pressurized compared with the atmospheric pressure. As a result, the liquid 32 contained in the inner space 30 a of the liquid tank 30 is pressurized, and discharged to the outside of the liquid tank 30 from the liquid discharge opening 30 b.

In a liquid pump having a configuration shown in FIG. 8, the first space 16 is opened to the atmosphere, and the second space 18 is in communication with the inner space 30 a of the liquid tank 30 via a flow path 52. Moreover, a liquid suction opening 30 c is provided in the liquid tank 30, and the inner space 30 a of the liquid tank 30 is in communication with the inside of a liquid container 31 disposed outside the liquid tank 30 via the liquid suction opening 30 c. The liquid 32 is contained in the liquid container 31. If the gas in the second space 18 is depressurized by the thermal transpiration flow 40 in the porous membrane 10 compared with the atmospheric pressure, the inner space 30 a of the liquid tank 30 which is in communication with the second space 18 via the flow path 52 is depressurized compared with the atmospheric pressure. As a result, the liquid 32 in the liquid container 31 is sucked into the inner space 30 a of the liquid tank 30 from the liquid suction opening 30 c.

In a liquid pump having a configuration shown in FIG. 9A, the first space 16 can be in communication with a flow path 53 via the flow path 51, the second space 18 can be in communication with the flow path 53 via the flow path 52, and the flow path 53 is in communication with the inner space 30 a of the liquid tank 30. Moreover, a liquid transfer opening 30 d is provided in the liquid tank 30, and the inner space 30 a of the liquid tank 30 is in communication with the inside of the liquid container 31 disposed outside the liquid tank 30 via the liquid transfer opening 30 d. The flow path 51 is provided with a first valve 61 which permits or cuts off the communication between the first space 16 and the flow path 53 (the inner space 30 a of the liquid tank 30). The first valve 61 comprises a three-way valve. When the communication between the first space 16 and the flow path 53 is cut off by the first valve 61, the first space 16 is opened to the atmosphere. The flow path 52 is provided with a second valve 62 which permits or cuts off the communication between the second space 18 and the flow path 53 (the inner space 30 a of the liquid tank 30). The second valve 62 comprises a three-way valve. When the communication between the second space 18 and the flow path 53 is cut off by the second valve 62, the second space 18 is opened to the atmosphere.

As shown in FIG. 9A, the communication between the first space 16 and the flow path 53 is permitted by the first valve 61, and the communication between the second space 18 and the flow path 53 is cut off by the second valve 62 (the second space 18 is opened to the atmosphere). In this case, if the gas in the first space 16 is pressurized by the thermal transpiration flow 40 in the porous membrane 10 compared with the atmospheric pressure, the inner space 30 a of the liquid tank 30 which is in communication with the first space 16 via the flow paths 51 and 53 is pressurized compared with the atmospheric pressure. As a result, the liquid 32 contained in the inner space 30 a of the liquid tank 30 is pressurized, and discharged into the liquid container 31 outside the liquid tank 30 from the liquid transfer opening 30 d.

On the other hand, as shown in FIG. 9B, the communication between the first space 16 and the flow path 53 is cut off by the first valve 61 (the first space 16 is opened to the atmosphere), and the communication between the second space 18 and the flow path 53 is permitted by the second valve 62. In this case, if the gas in the second space 18 is depressurized by the thermal transpiration flow 40 in the porous membrane 10 compared with the atmospheric pressure, the inner space 30 a of the liquid tank 30 which is in communication with the second space 18 via the flow paths 52 and 53 is depressurized compared with the atmospheric pressure. As a result, the liquid 32 contained in the liquid container 31 is sucked into the inner space 30 a of the liquid tank 30 from the liquid transfer opening 30 d. Thus, the liquid pump having a configuration shown in FIG. 9A has a discharge unit to discharge the liquid 32 in the inner space 30 a of the liquid tank 30 from the liquid transfer opening 30 d, and a suction unit to suck the liquid 32 into the inner space 30 a of the liquid tank 30 from the liquid transfer opening 30 d. If the liquid level in the liquid tank 30 is lower than the liquid level in the liquid container 31, the first space 16 is opened to the atmosphere (the communication between the first space 16 and the flow path 53 is cut off) by the first valve 61, and the second space 18 is opened to the atmosphere (the communication between the first space 16 and the flow path 53 is cut off). As a result, the liquid 32 in the liquid container 31 can be transferred to the inner space 30 a of the liquid tank 30 by the principle of a siphon.

The liquid pump according to the embodiment described above can transfer the liquid 32 by using the thermal transpiration flow 40 in the porous membrane 10. This liquid pump does not pulsate the liquid 32 in contrast with a liquid pump such as a gear pump, and can considerably reduce vibration and noise. Moreover, the liquid 32 does not contact a pump mechanism, and there is therefore no such problem as erosion. Consequently, the liquid pump according to the embodiment described above can be used for various purposes.

As described above, the maximum value of the pressure difference produced by the thermal transpiration flow shown in Nonpatent Document 1 is 1.05 kPa. Even if this pressure difference is used to transfer a liquid, it is possible to only obtain performance that achieves a pump head of about 10 cm (a pump head of 1 mm per pressure difference of 10 Pa), so that adequate performance of a liquid pump cannot be obtained. In contrast, according to the present embodiment, a pressure difference of 25.5 kPa can be obtained between the first space 16 and the second space 18 by the thermal transpiration flow 40, and a performance that achieves a pump head of 2.55 m, sufficient for a liquid pump, can be obtained.

The relation between energy applied to the front surface 10 a of the porous membrane 10 and the transfer flow volume of the liquid 32 in the liquid pump according to the embodiment described above is shown in FIG. 10. As shown in FIG. 10, the transfer flow volume of the liquid 32 is higher when the energy applied to the front surface 10 a of the porous membrane 10 is higher. Therefore, the transfer flow volume of the liquid 32 can be adjusted by the adjustment of the energy applied to the front surface 10 a of the porous membrane 10.

The devices for bringing the temperature of the front surface 10 a of the porous membrane 10 to a temperature higher than the temperature of the rear surface 10 b to generate the thermal transpiration flow 40 in the porous membrane 10 in the liquid pump for transferring the liquid 32 is not limited to the emission of the electromagnetic waves (light) 44 from the light source 24. It is also possible to use other heat sources to heat the front surface 10 a of the porous membrane 10 so that the temperature of the front surface 10 a of the porous membrane 10 will be higher than the temperature of the rear surface 10 b. When, for example, exhaust heat of equipment is used as a heat source to heat the front surface 10 a of the porous membrane 10, the liquid 32 can be transferred without the use of electricity. Therefore, the liquid pump can be used even in an isolated space that a power transmission line cannot reach, for example, in a spacecraft (satellite), in a ship, in a deep-sea craft, in a deep-sea building, in an underground structure, in an automobile (electric vehicle in particular), in a tunnel, or in a human body. There are also advantages when the liquid pump is used in a range that can be reached by a power transmission line; for example, no external power supply is needed to transfer the liquid 32, and the transfer of the liquid 32 is automatically started when the heat source reaches a high temperature. It is also possible to apply the transfer of the liquid 32 by the liquid pump to, for example, the circulation of cooling water, the circulation of an air-conditioner refrigerant, and the propulsion of electric power generation or a movable body.

In a liquid pump having a configuration shown in FIG. 11, a plurality of liquid tanks 30-1 to 30-3 are provided in parallel. The first space 16 can be in communication with flow paths 53-1 to 53-3 via the flow path 51, the second space 18 can be in communication with the flow paths 53-1 to 53-3 via the flow path 52, and the flow paths 53-1 to 53-3 can be in communication with inner spaces 30 a-1 to 30 a-3 of the liquid tanks 30-1 to 30-3, respectively. Moreover, liquid transfer openings 30 d-1 to 30 d-3 are respectively provided in the liquid tanks 30-1 to 30-3, and the inner spaces 30 a-1 to 30 a-3 in the liquid tanks 30-1 to 30-3 can be in communication with the inside of the liquid container 31 via the liquid transfer openings 30 d-1 to 30 d-3. The flow paths 53-1 to 53-3 are respectively provided with on-off valves 63-1 to 63-3 which permit or cut off the communication between the flow paths 51 and 52 and the inner spaces 30 a-1 to 30 a-3 of the liquid tanks 30-1 to 30-3. The liquid transfer openings 30 d-1 to 30 d-3 are respectively provided with on-off valves 64-1 to 64-3 which permit or cut off the communication between the inside of the liquid container 31 and the inner spaces 30 a-1 to 30 a-3 of the liquid tanks 30-1 to 30-3. Liquids 32-1 to 32-3 of different kinds are contained in the inner spaces 30 a-1 to 30 a-3 of the liquid tanks 30-1 to 30-3, respectively.

According to the configuration example shown in FIG. 11, the liquids 32-1 to 32-3 of different kinds can be transferred by opening and closing the on-off valves 63-1 to 63-3 and 64-1 to 64-3. For example, when the on-off valves 63-2 and 64-2 are opened and the on-off valves 63-1, 63-3, 64-1, and 64-3 are closed as shown in FIG. 11, the communication between the flow paths 51 and 52 and the inner space 30 a-2 of the liquid tank 30-2 and the communication between the inside of the liquid container 31 and the inner space 30 a-2 of the liquid tank 30-2 are permitted, while the communication between the flow paths 51 and 52 and the inner spaces 30 a-1 and 30 a-3 of the liquid tanks 30-1 and 30-3 and the communication between the inside of the liquid container 31 and the inner spaces 30 a-1 and 30 a-3 of the liquid tanks 30-1 and 30-3 are cut off. In this case, the liquid 32-2 contained in the inner space 30 a-2 of the liquid tank 30-2 can be transferred. When the communication between the first space 16 and the flow path 53-2 is permitted by the first valve 61 and the communication between the second space 18 and the flow path 53-2 is cut off by the second valve 62 (the second space 18 is opened to the atmosphere) as shown in FIG. 11, the gas in the first space 16 is pressurized by the thermal transpiration flow 40 in the porous membrane 10 compared with the atmospheric pressure, so that the liquid 32-2 in the inner space 30 a-2 of the liquid tank 30-2 is discharged into the liquid container 31 from the liquid transfer opening 30 d-2. On the other hand, when the communication between the first space 16 and the flow path 53-2 is cut off by the first valve 61 (the first space 16 is opened to the atmosphere) and the communication between the second space 18 and the flow path 53-2 is permitted by the second valve 62, the gas in the second space 18 is depressurized by the thermal transpiration flow 40 in the porous membrane 10 compared with the atmospheric pressure, so that the liquid 32-2 in the liquid container 31 is sucked into the inner space 30 a-2 of the liquid tank 30-2 from the liquid transfer opening 30 d-2.

The liquid pump described above can be used to configure a cooling device. In a cooling device having a configuration shown in FIG. 12, the inner space 30 a of the liquid tank 30 is in communication with a cooling flow path 54 passing in the vicinity of a high-temperature object (cooling target) 60 via the liquid transfer opening 30 d, and an inner space 50 a of a liquid tank 50 is in communication with the cooling flow path 54 via a liquid transfer opening 50 d. Moreover, the inner space 50 a of the liquid tank 50 is opened to the atmosphere via an atmospheric open opening 50 e. The liquid tanks 30 and 50 are respectively provided with cooling fins 30 f and 50 f, and a radiator 57 is provided in a flow path 55 which connects the cooling flow path 54 to the liquid transfer opening 30 d of the liquid tank 30. The cooling liquid 32 is contained in the inner spaces 30 a and 50 a of the liquid tanks 30 and 50.

If the communication between the first space 16 and the flow path 53-2 is permitted by the first valve 61 and the communication between the second space 18 and the flow path 53-2 is cut off by the second valve 62 (the second space 18 is opened to the atmosphere) as shown in FIG. 12, the gas in the first space 16 is pressurized by the thermal transpiration flow 40 in the porous membrane 10 compared with the atmospheric pressure, so that the liquid 32 contained in the inner space 30 a of the liquid tank 30 is pressurized, and the cooling liquid 32 is supplied to the cooling flow path 54 from the liquid transfer opening 30 d. As a result, the high-temperature object 60 can be cooled. The cooling liquid 32 which has passed through the cooling flow path 54 flows into the inner space 50 a of the liquid tank 50 from the liquid transfer opening 50 d.

When the liquid 32 in the inner space 30 a of the liquid tank 30 has decreased, the first valve 61 and the second valve 62 are changed as shown in FIG. 13. Thus, the communication between the first space 16 and the flow path 53-2 is cut off by the first valve 61 (the first space 16 is opened to the atmosphere), and the communication between the second space 18 and the flow path 53-2 is permitted by the second valve 62. In this case, the gas in the second space 18 is depressurized by the thermal transpiration flow 40 in the porous membrane 10 compared with the atmospheric pressure, so that the inner space 30 a of the liquid tank 30 is depressurized compared with the atmospheric pressure, and the cooling liquid 32 is sucked into the inner space 30 a of the liquid tank 30 from the liquid transfer opening 30 d. As a result, the liquid 32 contained in the inner space 50 a of the liquid tank 50 is supplied to the cooling flow path 54, and the high-temperature object 60 can be cooled.

When the liquid 32 in the inner space 50 a of the liquid tank 50 has decreased, the first valve 61 and the second valve 62 are changed as shown in FIG. 12. Thus, the communication between the first space 16 and the flow path 53-2 is permitted by the first valve 61, and the communication between the second space 18 and the flow path 53-2 is cut off by the second valve 62 (the second space 18 is opened to the atmosphere). If the operation described above is repeated, the thermal transpiration flow 40 in the porous membrane 10 can be used to transfer the cooling liquid 32 back and forth between the inner space 30 a of the liquid tank 30 and the inner space 50 a of the liquid tank 50 via the cooling flow path 54, and the high-temperature object 60 can be cooled. In order to generate the thermal transpiration flow 40 in the porous membrane 10, it is also possible to use heat energy generated by the high-temperature object 60 to heat the front surface 10 a of the porous membrane 10, and thereby bring the temperature of the front surface 10 a of the porous membrane 10 to a temperature higher than the temperature of the rear surface 10 b. In this case, when the high-temperature object 60 has reached a high temperature, the transfer of the liquid 32 is automatically started, and an abnormal temperature rise in the high-temperature object 60 can be automatically prevented.

The liquid pump described above can also be used to configure an electricity generating equipment. In an electricity generating equipment having a configuration shown in FIG. 14, a propeller (mill) 58 is provided in a flow path 56 which connects the liquid transfer opening 30 d of the liquid tank 30 to the liquid transfer opening 50 d of the liquid tank 50. The liquid 32 flows through the flow path 56, and the propeller 58 is thereby rotated to generate power. The propeller 58 is mechanically coupled to an electric generator 59, and the electric generator 59 generates electric power (generates electricity) by using the power resulting from the rotation of the propeller 58.

If the communication between the first space 16 and the flow path 53-2 is permitted by the first valve 61 and the communication between the second space 18 and the flow path 53-2 is cut off by the second valve 62 (the second space 18 is opened to the atmosphere) as shown in FIG. 14, the gas in the first space 16 is pressurized by the thermal transpiration flow 40 in the porous membrane 10 compared with the atmospheric pressure, so that the liquid 32 contained in the inner space 30 a of the liquid tank 30 is pressurized, and the liquid 32 is supplied to the flow path 56 (propeller 58) from the liquid transfer opening 30 d. As a result, the propeller 58 is rotationally driven to generate power, and the electric generator 59 generates electric power. The liquid 32 which has passed through the flow path 56 flows into the inner space 50 a of the liquid tank 50 from the liquid transfer opening 50 d.

When the liquid 32 in the inner space 30 a of the liquid tank 30 has decreased, the first valve 61 and the second valve 62 are changed as shown in FIG. 15. Thus, the communication between the first space 16 and the flow path 53-2 is cut off by the first valve 61 (the first space 16 is opened to the atmosphere), and the communication between the second space 18 and the flow path 53-2 is permitted by the second valve 62. In this case, the gas in the second space 18 is depressurized by the thermal transpiration flow 40 in the porous membrane 10 compared with the atmospheric pressure, so that the inner space 30 a of the liquid tank 30 is depressurized compared with the atmospheric pressure, and the cooling liquid 32 is sucked into the inner space 30 a of the liquid tank 30 from the liquid transfer opening 30 d. As a result, the liquid 32 contained in the inner space 50 a of the liquid tank 50 is supplied to the flow path 56, the propeller 58 is rotationally driven to generate power, and the electric generator 59 generates electric power.

When the liquid 32 in the inner space 50 a of the liquid tank 50 has decreased, the first valve 61 and the second valve 62 are changed as shown in FIG. 14. Thus, the communication between the first space 16 and the flow path 53-2 is permitted by the first valve 61, and the communication between the second space 18 and the flow path 53-2 is cut off by the second valve 62 (the second space 18 is opened to the atmosphere). If the operation described above is repeated, the thermal transpiration flow 40 in the porous membrane 10 can be used to transfer the liquid 32 back and forth between the inner space 30 a of the liquid tank 30 and the inner space 50 a of the liquid tank 50 via the flow path 56, power can be generated by the rotation of the propeller 58, and the electric generator 59 can generate electric power.

While the embodiment of the present invention has been described above, it should be understood that the present invention is not in the least limited to the above embodiment and various modifications can be made without departing from the spirit of the invention. 

What is claimed is:
 1. A pump comprising: a porous medium which has light absorption properties in a predetermined wavelength range and which has pores; and a heating unit which applies electromagnetic waves having a peak of radiant intensity in the predetermined wavelength range to a first surface of the porous medium to heat the first surface of the porous medium, wherein the first surface of the porous medium is in contact with a first space, a second surface of the porous medium different from the first surface is in contact with a second space, the first space is in communication with the second space via the pores, and a gas in the second space can be transferred to the first space via the communication pores.
 2. The pump according to claim 1, wherein the pores of the porous medium have a diameter equal to or less than a length that is five times the mean free path of the gas filling the first space or the second space.
 3. The pump according to claim 1, wherein the electromagnetic waves are applied to the first surface of the porous medium via a transmission window having light transmission properties in the predetermined wavelength range.
 4. The pump according to claim 1, further comprising a heat radiation device for heat radiation in the second surface of the porous medium.
 5. The pump according to claim 1, wherein the porous medium has a peak of light absorption properties for infrared light, and the heating unit applies electromagnetic waves having a peak of radiant intensity in an infrared light region to the first surface of the porous medium.
 6. The pump according to claim 1, wherein the porous medium has a peak of light absorption properties for visible light, and the heating unit applies electromagnetic waves having a peak of radiant intensity in a visible light region to the first surface of the porous medium.
 7. The pump according to claim 1, wherein the porous medium is made of silicon dioxide, and the predetermined wavelength range is 2.3 μm or more.
 8. The pump according to claim 1, wherein the porous medium is made of silicon dioxide, and the predetermined wavelength range is 2.3 to 4.0 μm.
 9. The pump according to claim 1, wherein the porous medium is made of silicon dioxide, and the predetermined wavelength range is 2.5 to 3.8 μm or is 4.8 μm or more.
 10. The pump according to claim 1, wherein gas molecules in the second space are transferred to the first space by a thermal transpiration flow generated in the pores.
 11. An actuator comprising a drive unit based on a pressure difference produced between the first space and the second space by the pump according to claim
 1. 12. The pump according to claim 1, wherein the first space is in communication with a third space having a liquid discharge opening, the pump being configured to pressurize the third space to discharge a liquid in the third space from the liquid discharge opening.
 13. The pump according to claim 1, wherein the second space is in communication with a fourth space having a liquid suction opening, the pump being configured to depressurize the fourth space to suck a liquid into the fourth space from the liquid suction opening.
 14. The pump according to claim 1, further comprising: a first valve which permits or cuts off the communication between a fifth space having a liquid transfer opening and the first space; and a second valve which permits or cuts off the communication between the fifth space and the second space, the pump having a discharge unit to discharge a liquid in the fifth space from the liquid transfer opening by using the first valve to permit the communication between the fifth space and the first space and using the second valve to cut off the communication between the fifth space and the second space, and a suction unit to suck a liquid into the fifth space from the liquid transfer opening by using the first valve to cut off the communication between the fifth space and the first space and using the second valve to permit the communication between the fifth space and the second space.
 15. A pump comprising: a porous medium having pores; and a device which brings the temperature of a first surface of the porous medium to a temperature higher than the temperature of a second surface of the porous medium different from the first surface, wherein the first surface of the porous medium is in contact with a first space, the second surface of the porous medium different from the first surface is in contact with a second space, the first space is in communication with the second space via the pores, a gas in the second space can be transferred to the first space via the communication pores, the first space is in communication with a third space having a liquid discharge opening, and a liquid in the third space can be discharged from the liquid discharge opening by the pressurization of the third space.
 16. A pump comprising: a porous medium having pores, and a device which brings the temperature of a first surface of the porous medium to a temperature higher than the temperature of a second surface of the porous medium different from the first surface, wherein the first surface of the porous medium is in contact with a first space, the second surface of the porous medium different from the first surface is in contact with a second space, the first space is in communication with the second space via the pores, a gas in the second space can be transferred to the first space via the communication pores, the second space is in communication with a fourth space having a liquid suction opening, and a liquid can be sucked into the fourth space from the liquid suction opening by the depressurization of the fourth space.
 17. An electricity generating equipment configured to generate electricity by using the pump according to claim
 1. 18. An electricity generating equipment configured to generate electricity by using the pump according to claim
 15. 19. An electricity generating equipment configured to generate electricity by using the pump according to claim
 16. 20. The pump according to claim 1, wherein the first surface is one main surface of the porous medium, and the second surface is a surface of the porous medium opposite to the one main surface.
 21. The pump according to claim 15, wherein the first surface is one main surface of the porous medium, and the second surface is a surface of the porous medium opposite to the one main surface.
 22. The pump according to claim 16, wherein the first surface is one main surface of the porous medium, and the second surface is a surface of the porous medium opposite to the one main surface. 